Ischemic effect of microembolization was studied in cats correlating several parameters of injury. In the same or directly adjacent brain sections the permeability of the blood-brain barrier (BBB) to proteins and water content of the tissues were compared. Immunocytochemical (PAP) methods revealed a close correlation between the extravasated serum proteins and the edematous changes evaluated by the specific gravity measurements. Immunocytochemical staining for the glial fibrillary acidic protein (GFAP) demonstrated a prompt fibrillary reaction of the astrocytes occurring as early as 10 minutes after injection of microspheres.